1. Field of the Invention
This invention relates to a flat coreless vibrator motor suitable for use as a silent alerting source for a portable card-type device.
2. Description of the Prior Art
There is a conventional vibrator motor in which an eccentric plate is mounted on an output shaft of a rotary motor of the ordinary type. Such a conventional vibrator motor is disclosed, for example, in Japanese Patent Application Laid-Open No. 63-290153. However, it is difficult to make such a vibrator motor thin because of the provision of the eccentric plate.
There is a conventional flat coreless vibrator motor in which one of three armature coils arranged on a rotor at an equal angular interval, i.e. pitch angle of 120.degree., is removed. Such a vibration motor is disclosed in the above Japenese Patent Application Laid-Open No. 63-290153. However, this conventional vibrator motor has a problem in that since there are only two armature coils, those portions which contribute to the production of torque cannot be sufficiently large, and therefore in order to obtain the required torque, a large amount of current is required. Another problem is that the rotor, having the two armature coils angularly spaced 120.degree. from each other, can not provide a sufficient weight unbalance which contributes to the production of vibrations.
There is known another flat coreless vibrator motor in which one of three armature coils arranged on a rotor at an equal angular interval, i.e. pitch angle of 120.degree., is smaller in size than the other two. Such a vibrator motor is disclosed in U.S. Pat. No. 4,864,276. However, in this conventional vibrator motor, since the rotor, having the three armature coils (one of which is smaller in size) angularly spaced 120.degree. from one another, has a disk-shaped configuration, weight unbalance of the rotor which contributes to the production of vibration is small. As a result, the amplitude of the vibration produced during the rotation of the rotor is small, and the vibration frequency becomes high. Therefore, when such a vibrator motor is used as an alerting source for a portable card-type device, users of the device feel unpleasant.